Methods of producing carbon fiber bundles from polyacrylonitrile-based precursor fiber bundles with a polymer in which acrylonitrile is the main component as a raw material are widely known, and are produced by first flameproofing at 200 to 300° C. in air or another oxidizing gas atmosphere to make a flameproof fiber bundle, and then carbonizing this at 800 to 2,000° C. in an inert gas atmosphere such as nitrogen or argon. In addition, graphitization is further performed in an inert gas atmosphere of at least 2,000° C., and producing graphite fiber bundles of much higher modulus of elasticity has been taking place.
With the object of efficiently producing and providing high-quality, high-performance carbon fiber bundles, a carbon fiber bundle is proposed having a strand strength of at least 389 kgf/mm2, made by calcining an acrylic precursor fiber bundle constituted from a copolymer made by copolymerizing 1 to 10% of a specific polymerizable unsaturated carboxylic acid alkyl ester and 0.25 to 5% of a specific polymerizable unsaturated carboxylic acid, and having a single thread inner/outer oxygen concentration ratio obtained by secondary ion mass spectrometry (SIMS) of no more than 6, or H/C of the inner layer of no more than 0.03, after flameproofing for 5 minutes at 260° C., further for 5 minutes at 280° C. in air at ambient pressure. The Examples propose carbon fiber bundles having a strand strength of 501 kgf/mm2 and strand modulus of elasticity of 26 tonf/mm2 with carbon fiber bundles from flameproofing, using 2.0 denier polyacrylonitrile-based precursor fiber bundles produced from a copolymer made from 92.5% acrylonitrile, 1.5% itaconic acid and 6% normal butyl methacrylate, these fibers for 30 minutes in air at 240 to 260° C., and heat treating in a nitrogen flow at up to 1,300° C. This succeeds at the production of carbon fiber bundles having relatively high tensile strength with a flameproofing time of a short time, even with fibers having large relative fineness of single fibers, by causing the flameproofing reactivity to decline by drastically lowering the acrylonitrile ratio.
In addition, Patent Documents 2 and 3 propose a method of efficiently producing thick fineness carbon fiber bundles by spinning a flameproof polymer with polyacrylonitrile-based polymer as a precursor to obtain a flameproof fiber bundle such that the single fiber fineness is at least 2 dtex, followed by carbonization treating this flameproof fiber bundle, and a carbon fiber bundle having a strand tensile strength of at least 4 GPa and strand tensile modulus of elasticity of at least 200 GPa by way of heat treating a polyacrylonitrile-based precursor fiber bundle in the liquid phase to obtain a flameproof fiber bundle, followed by carbonization treating this flameproof fiber bundle.
On the other hand, as a means for increasing the production output in the field of composite fibers, a method has been known of thickening a fiber bundle by increasing the number of fibers or thickening the single fiber fineness, and thus increasing the discharge amount per spinneret. If thickening the fiber bundle in this way, the production output will increase, while due to being related with a cost reduction at the same time due to an increase in cost of equipment being curbed to a minimum, it has been widely used in major industrial fibers such as polyester and nylons (refer to Patent Document 2).    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H09-31758    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2008-202207    Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2004-300600